XML-RPC is a specification that enables communication between WordPress and other systems. This was achieved by standardizing this communication using HTTP as the transport mechanism and XML as the encoding mechanism.

XML-RPC is largely out of date and should be disabled:

https://noprob.it/knowhow/deaktivieren-der-xmlrpc-schnittstelle/

Source: https: //bestwebsoft.com/what-is-xml-rpc/